Clearing ladder fuels is a key element to defensible space. Just as forests have plants at different levels — canopy, understory, and forest floor — they also have potential wildfire fuels at these varied levels. It’s when these fuels come into contact with one another, that fires can climb to the tops of trees, creating potential for destructive crowning.
“Ladder fuels are live and dead fuels that have the potential to promote active fire to transfer from a surface, such as the forest floor, to the crown (or top) of a tree,” U.S. Forest Service Lake Tahoe Basin Management Unit Fuels Battalion Chief Kenneth Heald explained in an email to Moonshine Ink. “Ladder fuels, such as tall grasses, shrubs, and tree branches give fire the ability to climb up the tree similar to climbing up a ladder.”
The U.S. Forest Service defines the three levels of naturally occurring fuels as aerial, surface, and ground.
Aerial fuels include things like branches, bark, and leaves that are at least 39 inches (1 meter) above the ground level. Below that comes surface fuels — items on the ground such as bushes, fallen leaves, stumps, and logs. At the lowest level is where ground fuels can be found. Not to be confused with surface fuels, ground fuels are what lie beneath the surface: decaying leaves and pine needles, roots, and rotting branches.
Ground fires tend to smolder without true flames exacerbating the spread. The danger comes, however, when the three types of fuel come into contact with one another.
You’ve likely heard about fires “crowning” as they spread through the forest. According to the forest service, this is the most destructive form of wildfire spread.
“Crowning is the movement of fire through the tops of trees or shrubs,” said Heald. He noted that there are two types of crown fires.
Passive crown fire occurs in areas where surface fire intensity is sufficient enough to ignite tree crowns, be it either a single tree or groups of trees. Active crown fires run through the tops of trees, independent of surface fire, Heald explained.
“Ladder fuels allow a surface fire to transition vertically into the crowns of trees, where ground-based fire resources are less effective in containing the fire,” Heald said. “And they promote spot fires that allow fires to escape containment.”
Clearing ladder fuels is a crucial part of defensible space clean-up. A prime example of a ladder fuel is a standing dead tree with dry, brittle branches running the vertical length of the trunk from the bottom up. Pruning and removing the lower limbs of a tree — on both live and dead wood — reduce ladder fuels. This, says Cal Fire, is frequently done alongside roads, which “increases the effectiveness of the road as an existing fuel break.” The North Lake Tahoe Fire Protection District recommends clearing shrubs and trees growing under the drip lines of trees, in addition to removing low-hanging branches below the bottom one-third of a tree’s height. Shrubs and young trees should also be cleared from beneath the canopies of larger trees.
“Reducing ladder fuels helps keep fires on the forest floor where suppression crews have a better ability to contain and control the fires,” Heald said. “[This] makes for a safer work environment overall for firefighters and lessens the impacts of fires on all resources — in communities along with the natural and cultural resources on our landscapes.”